Pattern formation in a secondary field: a hierarchy of regulatory genes subdivides the developing Drosophila wing disc into discrete subregions

Controlling growth of the wing: vestigial integrates signals from the compartment boundaries

In the past few years it has become apparent that the anterior/posterior (A/P) and dorsal/ventral (D/V) compartment boundaries serve as the source of long-range signals that organize the A/P and D/V axes of the Drosophila wing. Recent work suggests that the vestigial gene may function as a nodal point through which the growth-controlling activity of these two patterning systems is integrated.

Organizing spatial pattern in limb development

Recent studies on the development of the legs and wings of Drosophila have led to the conclusion that insect limb development is controlled by localized pattern organizing centers, analogous to those identified in vertebrate embryos. Genetic analysis has defined the events that lead to the formation of these organizing centers and has led to the identification of gene products that mediate organizer function. The possibility of homology between vertebrate and insect limbs is considered in light of recently reported similarities in patterns of gene expression and function.

A hierarchy of cross-regulation involving Notch, wingless, vestigial and cut organizes the dorsal/ventral axis of the Drosophila wing

Short-range interaction between dorsal and ventral cells establishes an organizing center at the dorsal/ventral compartment boundary that controls growth and patterning of the wing. We report here that the dorsal/ventral organiser is built though a hierarchy of regulatory interactions involving the Notch and wingless signal transduction pathways and the vestigial gene. wingless and vestigial are activated in cells adjacent to the dorsal/ventral boundary by a Notch-dependent signal. vestigial is initially expressed under control of an early dorsal/ventral boundary enhancer that does not depend on wingless activity. Similarly, activation of wingless does not require vestigial function, showing that wingless and vestigial are parallel targets of the Notch pathway. Subsequently, vestigial is expressed in a broad domain that fills the wing pouch. This second phase of vestigial expression depends on Wingless function in cells at the dorsal/ventral boundary. In addition, the Notch and Wingless pathways act synergistically to regulate expression of cut in cells at the dorsal/ventral boundary. Thus Wingless can act locally, in combination with Notch, to specify cell fates, as well as at a distance to control vestigial expression. These results suggest that secreted Wingless protein mediates both long-range and short-range patterning activities of the dorsal/ventral boundary.

Integration of positional signals and regulation of wing formation and identity by Drosophila vestigial gene

Appendage formation is organized by signals from discrete sources that presumably act upon downstream genes to control growth and patterning. The Drosophila vestigial gene is selectively required for wing-cell proliferation, and is sufficient to induce outgrowths of wing tissue from eyes, legs and antennae. Different signals activate separate enhancers to control vestigial expression: first, in the dorsal/ventral organizer through the Notch pathway, and subsequently, in the developing wing blade by decapentaplegic and a signal from the dorsal/ventral organizer. Signal integration must be a general feature of genes like vestigial, that regulate growth or patterning along more than one axis.

Specification of the wing by localized expression of wingless protein

Limb development in Drosophila depends on subdivision of the limb primordia into functional units called compartments. Cell interactions across compartment boundaries establish pattern-organizing centres that control growth and specify cell fates along the anteroposterior (AP) and dorsoventral (DV) axes of the limbs. AP subdivision of the disc primordia is inherited from the embryonic ectoderm. DV subdivision of the wing disc occurs during the second larval instar through localized expression of the apterous protein (Apterous) in dorsal cells. A third major subdivision of the wing disc into wing and body-wall compartments also occurs in the second instar. Here we show that specification of the wing primordium in early second instar depends on activity of the AP patterning system but not the DV system. These results define two distinct roles for the wingless gene: a primary role in specifying the wing primordium, and a subsequent role mediating the patterning activities of the DV compartment boundary.

Control of the gene optomotor-blind in Drosophila wing development by decapentaplegic and wingless

Diffusible factors of several protein families control appendage outgrowth and patterning in both insects and vertebrates. In Drosophila wing development, the gene decapentaplegic (dpp) is expressed along the anteroposterior compartment boundary. Early wingless (wg) expression is involved in setting up the dorsoventral boundary. Interaction between dpp- and wg-expressing cells promotes appendage outgrowth. Here, it is shown that optomotor-blind (omb) expression is required for distal wing development and is controlled by both dpp and wg. Ectopic omb expression can lead to the growth of additional wings. Thus, omb is essential for wing development and is controlled by two signaling pathways.

Delta is a ventral to dorsal signal complementary to Serrate, another Notch ligand, in Drosophila wing formation

Wing margin formation in Drosophila requires the Notch receptor and, in the dorsal compartment, one of its ligands, Serrate. We provide evidence that Delta, the other known ligand for Notch, is also essential for this process. Delta is required in ventral cells at the dorsal/ventral compartment boundary, where its expression is specifically elevated in second-instar wing discs during wing margin formation. Moreover, ectopic Delta expression induces wingless, vestigial, and cut and causes adult wing tissue outgrowth in the dorsal compartment. The effect is mediated by Notch, because loss of Notch activity suppresses Delta-induced ectopic wing outgrowth. Whereas ectopic expression of Notch or the truncated activated Notch induces cut in both dorsal and ventral compartments, ectopic Delta expression induces cut only in the dorsal compartment and ectopic Serrate induces cut only in the ventral compartment. These observations indicate that Notch-expressing cells in a given compartment have different responses to Delta and Serrate. We propose that Delta and Serrate function as compartment-specific signals in the wing disc, to activate Notch and induce downstream genes required for wing formation.

Activation and function of Notch at the dorsal-ventral boundary of the wing imaginal disc

The cells along the dorsoventral boundary of the Drosophila wing imaginal disc have distinctive properties and their specification requires Notch activity. Later in development, these cells will form the wing margin, where sensory organs and specialised trichomes appear in a characteristic pattern. We find that Notch is locally activated in these cells, as demonstrated by the restricted expression of the Enhancer of split proteins in dorsal and ventral cells abutting the D/V boundary throughout the third larval instar. Furthermore other genes identified by their involvement in Notch signaling during neurogenesis, such as Delta and Suppressor of Hairless, also participate in Notch function at the dorsoventral boundary. In addition, Serrate, a similar transmembrane protein to Delta, behaves as a ligand required in dorsal cells to activate Notch at the boundary. Notch gain-of-function alleles in which Notch activity is not restricted to the dorsoventral boundary cause miss-expression of cut and wingless and overgrowth of the disc, illustrating the importance of localised Notch activation for wing development.

Serrate signals through Notch to establish a Wingless-dependent organizer at the dorsal/ventral compartment boundary of the Drosophila wing

Growth and patterning of the Drosophila wing is controlled by organizing centers located at the anterior-posterior and dorsal-ventral compartment boundaries. Interaction between cells in adjacent compartments establish the organizer. We report here that Serrate and Notch mediate the interaction between dorsal and ventral cells to direct localized expression of Wingless at the D/V boundary. Serrate serves as a spatially localized ligand which directs Wg expression through activation of Notch. Ligand independent activation of Notch is sufficient to direct Wg expression, which in turn mediates the organizing activity of the D/V boundary.

Serrate and wingless cooperate to induce vestigial gene expression and wing formation in Drosophila

BACKGROUND

The appendages of insects, like the limbs of vertebrates, grow out of the body wall after the establishment of a proximo-distal axis among a group of primordial cells. In Drosophila, the wing develops in the limbless larva from one of the imaginal discs of the thorax, which give rise to the adult epidermis. The earliest identified requirement in wing development is for the induction of vestigial (vg) gene expression at the interface between ventral cells and dorsal cells of the wing disc. It has been proposed that this event requires two reciprocal signals--one from the dorsal to the ventral cells and the other from the ventral to the dorsal cells--which trigger vg expression at the presumptive wing margin and hence initiate the development of the wing tissue.

RESULTS

We have identified four genes--Serrate (Ser), wingless (wg), Notch and Suppressor of Hairless (Su(H))--whose activity is required during the second and early third larval instars for the expression of vg. Analysis of the functions and patterns of expression of these genes at the time of the inductive event indicates that the Ser protein acts as a dorsal signal, and the Wg protein as a ventral signal for the induction of vg expression. Furthermore, the expression of both Ser and Wg is sufficient to trigger ectopic wing development in the wing disc and leg discs. The product of the Notch gene, which encodes a receptor, is also required for this event and we suggest that its role is to integrate the inputs of Ser and Wg.

CONCLUSIONS

We show that the induction of vg, which initiates wing development in Drosophila, requires the combined activities of Ser, wg and Notch. Based on the patterns of expression and requirements for Ser and wg in this process, we propose that Ser is a dorsal signal and that Wg is a ventral signal, and that their combination at the dorso-ventral interface activates the Notch receptor and leads to vg expression.

Origin of bilaterian body plans: evolution of developmental regulatory mechanisms

An argument is proposed to explain the origin of large metazoans, based on the regulatory processes that underlie the morphogenetic organization of pattern in modern animals. Genetic regulatory systems similar to those used in modern, indirectly developing marine invertebrates are considered to indicate the Precambrian regulatory platform on which were erected innovations that underlie the development of macroscopic body plans. Those systems are genetic regulatory programs that produce groups of unspecified "set-aside cells" and hierarchical regulatory programs that initially define regions of morphogenetic space in terms of domains of transcription factor expression. These ideas affect interpretation of the development of arthropods and chordates as well as interpretation of the role of the genes of the homeotic complex in embryogenesis.

Induction of the LIM homeobox gene Lmx1 by WNT7a establishes dorsoventral pattern in the vertebrate limb

During vertebrate limb development, the ectoderm directs the dorsoventral patterning of the underlying mesoderm. To define the molecular events involved in this process, we have analyzed the function of WNT7a, a secreted factor expressed in the dorsal ectoderm, and LMX1, a LIM homeodomain transcription factor expressed in the dorsal mesenchyme. Ectopic expression of Wnt7a is sufficient to induce and maintain Lmx1 expression in limb mesenchyme, both in vivo and in vitro. Ectopic expression of Lmx1 in the ventral mesenchyme is sufficient to generate double-dorsal limbs. Thus, the dorsalization of limb mesoderm appears to involve the WNT7a-mediated induction of Lmx1 in limb mesenchymal cells.

Pattern formation in Drosophila head development: the role of the orthodenticle homeobox gene

Significant progress has been made towards understanding how pattern formation occurs in the imaginal discs that give rise to the limbs of Drosophila melanogaster. Here, we examine the process of regional specification that occurs in the eye-antennal discs, which form the head of the adult fruitfly. We demonstrate genetically that there is a graded requirement for the activity of the orthodenticle homeobox gene in forming specific structures of the developing head. Consistent with this result, we show that OTD protein is expressed in a graded fashion across the disc primordia of these structures and that different threshold levels of OTD are required for the formation of specific subdomains of the head. Finally, we provide evidence suggesting that otd acts through the segment polarity gene engrailed to specify medial head development.

wingless inhibits morphogenetic furrow movement in the Drosophila eye disc

Differentiation of the Drosophila eye imaginal disc is an asynchronous, repetitive process which proceeds across the disc from posterior to anterior. Its propagation correlates with the expression of decapentaplegic at the front of differentiation, in the morphogenetic furrow. Both differentiation and decapentaplegic expression are maintained by Hedgehog protein secreted by the differentiated cells posterior to the furrow. However, their initiation at the posterior margin occurs prior to hedgehog expression by an unknown mechanism. We show here that the wingless gene contributes to the correct spatial localization of initiation. Initiation of the morphogenetic furrow is restricted to the posterior margin by the presence of wingless at the lateral margins; removal of wingless allows lateral initiation. Ectopic expression of wingless at the posterior margin can also inhibit normal initiation. In addition, the presence of wingless in the center of the disc can prevent furrow progression. These effects of wingless are achieved without altering the expression of decapentaplegic.

Creating a Drosophila wing de novo, the role of engrailed, and the compartment border hypothesis

Anterior/posterior compartment borders bisect every Drosophila imaginal disc, and the engrailed gene is essential for their function. We analyzed the role of the engrailed and invected genes in wing discs by eliminating or increasing their activity. Removing engrailed/invected from posterior wing cells created two new compartments: an anterior compartment consisting of mutant cells and a posterior compartment that grew from neighboring cells. In some cases, these compartments formed a complete new wing. Increasing engrailed activity also affected patterning. These findings demonstrate that engrailed both directs the posterior compartment pathway and creates the compartment border. These findings also establish the compartment border as the pre-eminent organizational feature of disc growth and patterning.

groucho and hedgehog regulate engrailed expression in the anterior compartment of the Drosophila wing

Drosophila imaginal discs are divided into units called compartments. Cells belonging to the same compartment are related by lineage and express a characteristic set of ‘selector genes’. The borders between compartments act as organizing centres that influence cell growth within compartments. Thus, in the cells immediately anterior to the anterior-posterior compartment boundary the presence of the hedgehog product causes expression of decapentaplegic, which, in turn, influences the growth and patterning of the wing disc. The normal growth of the disc requires that posterior-specific genes, such as hedgehog and engrailed are not expressed in cells of the anterior compartment. Here we show that hedgehog can activate engrailed in the anterior compartment and that both hedgehog and engrailed are specifically repressed in anterior cells by the activity of the neurogenic gene groucho. In groucho mutant discs, hedgehog and engrailed are expressed at the dorsoventral boundary of the anterior compartment, leading to the ectopic activation of decapentaplegic and patched and to a localised increase in cell growth associated with pattern duplications. The presence of engrailed in the anterior compartment causes the transformation of anterior into posterior structures.

Cell recognition, signal induction, and symmetrical gene activation at the dorsal-ventral boundary of the developing Drosophila wing

Appendage formation in insects and vertebrates depends upon signals from both the anterior-posterior and dorsal-ventral (DV) axes. In Drosophila, wing formation is organized symmetrically around the DV boundary of the growing wing imaginal disc and requires interactions between dorsal and ventral cells. Compartmentalization of the wing disc, dorsal cell behavior, and the expression of two dorsally expressed putative signaling molecules, fringe (fng) and Serrate (Ser), are regulated by the apterous selector gene. Here, we demonstrate that fng and Ser have distinct roles in a novel cell recognition and signal induction process. fng serves as a boundary-determining molecule such that Ser is induced wherever cells expressing fng and cells not expressing fng are juxtaposed. Ser in turn triggers the expression of genes involved in wing growth and patterning on both sides of the DV boundary.

Involvement of Wnt-1 in the formation of the mes/metencephalic boundary

Wnt-1, a putative signaling molecule, is required before the 7 somite stage (E8.5) for the development of midbrain structures in the mouse. We show here that Wnt-1 is also needed for the formation of a boundary between the mesencephalic and metencephalic domains of the neural tube. In embryos homozygous for the Wnt-1sw allele, mesencephalic and metencephalic markers fail to segregate and the establishment of a straight limit of Otx-2 and Wnt-1 expression at the mid-hindbrain junction is impaired. In addition, as observed previously in heterotopic mes/metencephalic transplantation experiments in avian embryos, Wnt-1 expression is induced at the border of ectopic mes- and metencephalic islands observed in Wnt-1sw/sw mutants, suggesting that, in situ, interactions between mes- and metencephalic cells reinforce Wnt-1 expression at the boundary.

Notch regulates wingless expression and is not required for reception of the paracrine wingless signal during wing margin neurogenesis in Drosophila

In the developing wing margin of Drosophila, wingless is normally expressed in a narrow stripe of cells adjacent to the proneural cells that form the sensory bristles of the margin. Previous work has shown that this wingless is required for the expression of the proneural achaete-scute complex genes and the subsequent formation of the sensory bristles along the margin; recently, it has been proposed that the proneural cells require the Notch protein to properly receive the wingless signal. We have used clonal analysis of a null allele of Notch to test this idea directly. We found that Notch was not required by prospective proneural margin cells for the expression of scute or the formation of sensory precursors, indicating Notch is not required for the reception of wingless signal. Loss of Notch from proneural cells produced cell-autonomous neurogenic phenotypes and precocious differentiation of sensory cells, as would be expected if Notch had a role in lateral inhibition within the proneural regions. However, loss of scute expression and of sensory precursors was observed if clones substantially included the normal region of wingless expression. These ‘anti-proneural’ phenotypes were associated with the loss of wingless expression; this loss may be partially or wholly responsible for the anti-proneural phenotype. Curiously, Notch- clones limited to the dorsal or ventral compartments could disrupt wingless expression and proneural development in the adjacent compartment. Analysis using the temperature-sensitive Notch allele indicated that the role of Notch in the regulation of wingless expression precedes the requirement for lateral inhibition in proneural cells. Furthermore, overexpression of wingless with a heat shock-wingless construct rescued the loss of sensory precursors associated with the early loss of Notch.

Shortsighted acts in the decapentaplegic pathway in Drosophila eye development and has homology to a mouse TGF-beta-responsive gene

Differentiation in the Drosophila eye imaginal disc traverses the disc as a wave moving from posterior to anterior. The propagation of this wave is driven by hedgehog protein secreted by the differentiated cells in the posterior region of the disc. Hedgehog induces decapentaplegic expression at the front of differentiation, in the morphogenetic furrow. We have identified a gene, shortsighted, which is expressed in a hedgehog-dependent stripe in the undifferentiated cells just anterior to the furrow and which appears to be involved in the transmission of the differentiation-inducing signal; a reduction in shortsighted function leads to a delay in differentiation and to a loss of photoreceptors in the adult. shortsighted is also required for a morphogenetic movement in the brain that reorients the second optic lobe relative to the first. shortsighted encodes a cytoplasmic leucine zipper protein with homology to a mouse gene, TSC-22, which is transcriptionally induced in response to TGF-beta.

frizzled regulates mirror-symmetric pattern formation in the Drosophila eye

Coordinated morphogenesis of ommatidia during Drosophila eye development establishes a mirror-image symmetric pattern across the entire eye bisected by an anteroposterior equator. We have investigated the mechanisms by which this pattern formation occurs and our results suggest that morphogenesis is coordinated by a graded signal transmitted bidirectionally from the presumptive equator to the dorsal and ventral poles. This signal is mediated by frizzled, which encodes a cell surface transmembrane protein. Mosaic analysis indicates that frizzled acts non-autonomously in an equatorial to polar direction. It also indicates that relative levels of frizzled in photoreceptor cells R3 and R4 of each ommatidium affect their positional fate choices such that the cell with greater frizzled activity becomes an R3 cell and the cell with less frizzled activity becomes an R4 cell. Moreover, this bias affects the choice an ommatidium makes as to which direction to rotate. Equator-outwards progression of elav expression and expression of the nemo gene in the morphogenetic furrow are regulated by frizzled, which itself is dynamically expressed about the morphogenetic furrow. We propose that frizzled mediates a bidirectional signal emanating from the equator.

Dorsal wing, a locus that affects dorsoventral wing patterning in Drosophila

The wing imaginal disc is subdivided into a dorsal and a ventral compartments. A new dominant homeotic mutation, Dorsal wing1 (Dlw1), transforms ventral into dorsal compartment in heterozygotes. This phenotype is similar to one of the dominant phenotypes of Polycomb (Pc) mutants. In Pc Dlw+/Pc+ Dlw1 double mutants, the transformation is greatly enhanced. The recessive phenotype of Dlw1 is the opposite to the dominant phenotype. Dlw1/Dlw1 somatic clones induced at any larval stage differentiate only ventral pattern on both wing surfaces. This effect is one of the somatic clone phenotypes of trithorax (trx) lethals. A similar dorsal-to-ventral transformation is observed in Pc Dlw/Dlw clones. Dlw1/Dlw1 clones have no effect elsewhere, except in the dorsal notum, which may differentiate extra macrochaetes. We propose that: (1) Dlw+ is required for the specification of dorsal compartment; (2) some genes of the Polycomb group act as negative regulators of Dlw+, while some genes of the trithorax group act as positive regulators.

Wingless induces transdetermination in developing Drosophila imaginal discs

Drosophila imaginal discs, the precursors of the adult fly appendages, have been the subject of intensive developmental studies, particularly on cell determination. Cultured disc fragments are recognized not only for the ability to maintain their determined state through extra cell divisions but also for the ability to transdetermine, or switch to the determined state of a different disc. An understanding of transdetermination at a molecular level will provide further insight into the requirements for maintaining cell determination. We find that ectopic expression of the Drosophila gene wingless induces transdetermination of foreleg imaginal disc cells to wing cells. This transdetermination occurs in foreleg discs of developing larvae without disc fragmentation. The in situ-transdetermining cells localize to the dorsal region of the foreleg disc. This wingless-induced transdetermination event is remarkably similar to the leg-to-wing switch that occurs after leg disc culture. Thus we have identified a new approach to a molecular dissection of transdetermination.

Compartments and appendage development in Drosophila

The appendages of Drosophila develop from the imaginal discs. During the extensive growth of these discs cell lineages are for the most part unfixed, suggesting a strong role for cell-cell interactions in controlling the final pattern of differentiation. However, during early and middle stages of development, discs are subdivided by strict lineage restrictions into a small number of spatially distinct compartments. These compartments appear to be maintained by stably inheriting states of gene expression; the compartment-specific expression of two such 'selector'-like genes, engrailed and apterous, are critical for anterior-posterior and dorso-ventral compartmentalization, respectively. Recent work suggests that one purpose of compartmentalization is to establish regions of specialized cells near compartment boundaries via intercompartmental induction, using molecules like the hedgehog protein. Thus, compartments can act as organizing centers for patterning within compartments. Evidence for non-compartmental patterning mechanisms will also be discussed.

Function of protein kinase A in hedgehog signal transduction and Drosophila imaginal disc development

Reduced protein kinase A (PKA) activity in anterior imaginal disc cells leads to cell-autonomous induction of decapentaplegic (dpp), wingless (wg), and patched (ptc) transcription that is independent of hedgehog (hh) gene activity. The resulting nonautonomous adult wing and leg pattern duplications are largely due to induced dpp and wg expression and resemble phenotypes elicited by ectopic hh expression. Inhibition of PKA in anterior cells close to the posterior compartment can substitute for hh activity to promote growth of imaginal discs, whereas overexpression of PKA can counteract transcriptional induction of ptc by hh in these cells. PKA therefore appears to be an integral component of the mechanism by which hh regulates the expression of key patterning molecules in imaginal discs.

Nubbin encodes a POU-domain protein required for proximal-distal patterning in the Drosophila wing

The nubbin gene is required for normal growth and patterning of the wing in Drosophila. We report here that nubbin encodes a member of the POU family of transcription factors. Regulatory mutants which selectively remove nubbin expression from wing imaginal discs lead to loss of wing structures. Although nubbin is expressed throughout the wing primordium, analysis of genetic mosaics suggests a localized requirement for nubbin activity in the wing hinge. These observations suggest the existence of a novel proximal-distal growth control center in the wing hinge, which is required in addition to the well characterized anterior-posterior and dorsal-ventral compartment boundary organizing centers.

Genetic and molecular studies of apterous: a gene implicated in the juvenile hormone system of Drosophila

The apterous (ap) gene in Drosophila melanogaster encodes a homeodomain transcription factor. It is required for the development of the wings and of a subset of embryonic muscles. The gene has been implicated in the juvenile hormone (JH) system because mutations in ap lead to JH deficiency, and are associated with defective histolysis of the larval fat body, arrested vitellogenesis, sterility, and aberrant sexual behavior, all of which are dependent on JH. We describe here the use of hemizygotes and germ-line clones, of X-ray- and hybrid dysgenesis-induced lethal ap alleles to determine the primary role of the gene during development. We find that ap lethality is polyphasic, but occurs primarily at the larval and pupal stages. The lethal phenotype is not associated with any overt morphological abnormality, suggesting that death occurs from a systemic malfunction. Strong interallelic complementation for the wing phenotype was found between some ap mutations induced by X-rays or by hybrid-dysgenesis. By Northern blot analysis, we demonstrate an increase in ap expression in pupae and adults as compared to embryos and larvae, suggesting that it is developmentally regulated. Finally, primer extension is used to determine the transcription start site of the gene.

Ectopic induction and reorganization of Wnt-1 expression in quail/chick chimeras

When grafted ectopically into the diencephalon of a chick host embryo, a portion of met-mesencephalon straddling the met-mesencephalic constriction has the capacity to induce En-2 expression in the surrounding host tissue. Subsequently, tectal and cerebellar structures, composed of both host and grafted cells, are reconstructed in this ectopic location at the expense of the host diencephalon. Previous experiments indicated that the induction of En-2 was correlated with Wnt-1 expression within the graft. The aim of the present study was: (i) to determine whether Wnt-1 expression was spatially regulated within the graft, (ii) to investigate whether host Wnt-1-expressing cells were also involved in the ectopic met-mesencephalic development and, if so, (iii) to localize these Wnt-1-positive domains in relation to the patterning of the ectopically developing met-mesencephalic territory. We studied the expression profile of Wnt-1, in relation with that of other positional markers, in quail/chick chimeras where various portions of met-mesencephalon had been grafted into the diencephalon. We found that Wnt-1 expression was reorganized within the graft, and that it was also induced in the host in contact with the graft. Moreover, these ectopic expressions of Wnt-1, in both the grafted and the surrounding host tissues, were organized in concert to form a continuous positive line at the host/graft junction, the location of which depended on the precise origin of the graft. Finally, we found that this line was frequently located at the limit between territories expressing different positional markers. We propose that Wnt-1 expression is turned on at the junction between domains of different phenotypes, and may be used as a border to stabilize these adjacent differently committed territories.

fringe, a Boundary-specific signaling molecule, mediates interactions between dorsal and ventral cells during Drosophila wing development

Wing formation in Drosophila requires interactions between dorsal and ventral cells. We describe a new gene, fringe, which is expressed in dorsal cells and encodes for a novel protein that is predicted to be secreted. Wing margin formation and distal wing outgrowth can be induced by the juxtaposition of cells with and without fringe expression, whether at the normal wing margin, at the boundaries of fringe mutant clones in the dorsal wing, or at sites of fringe misexpression in the ventral wing. By contrast, both loss of fringe expression and uniform fringe expression cause wing loss. These observations suggest that fringe encodes a boundary-specific cell-signaling molecule that is responsible for dorsal-ventral cell interactions during wing development.

Induction of a mirror-image duplication of anterior wing structures by localized hedgehog expression in the anterior compartment of Drosophila melanogaster wing imaginal discs

The segment polarity gene hedgehog (hh) encodes a secretory protein involved in cell-cell communication in Drosophila melanogaster. The hh gene is expressed in the posterior compartment and is essential for the establishment and maintenance of the anterior/posterior-compartment boundary of each embryonic parasegment [Ingham, P.W., Nature 366 (1993) 560-562]. To clarify possible hh functions in adult appendage formation, we isolated a fly line (h9D) associated with a wing malformation from among fly lines with an hh transgene whose expression is under the control of trapped enhancers. In h9D flies, the ectopic expression of hh occurred in the anterior edge of wing pouch in the wing disc. This abnormal hh expression resulted in not only a mirror-image duplication and ectopic outgrowth in the anterior wing compartment, but also the ectopic expression of patched and decapentaplegic, strongly suggesting that the hh product serves as a morphogen or an inducer essential for wing development, including the proximal/distal axis formation.

Molecular biology of embryonic development: how far have we come in the last ten years?

The successes of molecular developmental biology over the last ten years have been particularly impressive in those directions favored by its major paradigms. New technologies have both guided and been guided by the progress of the field. I review briefly some of the major insights into embryonic development that have derived from research in four specific areas: early embryogenesis of various forms; 'pattern formation'; evolutionary conservation of regulatory elements; and spatial mechanisms of gene regulation. There remain many major problem areas, some of which may require new orientations to solve.

Blistered: a gene required for vein/intervein formation in wings of Drosophila

We have characterized the blistered (bs) locus phenotypically, genetically and developmentally using a set of new bs alleles. Mutant defects range from wings with ectopic veins and intervein blisters to completely ballooned wings where the distinction between vein and intervein is lost. Mosaic analyses show that severe bs alleles behave largely autonomously; homozygous patches having vein-like properties. Developmental analyses were undertaken using light and electron microscopy of wild-type and bs wings as well as confocal microscopy of phalloidin- and laminin-stained preparations. bs defects were first seen early in the prepupal period with the failure of apposition of dorsal and ventral wing epithelia. Correspondingly, during definitive vein/intervein differentiation in the pupal period (18-36 hours after puparium formation), the extent of dorsal/ventral reapposition is reduced in bs wings. Regions of the wing that fail to become apposed differentiate properties of vein cells; i.e. become constricted apically and acquire a laminin-containing matrix basally. To further understand bs function, we examined genetic interactions between various bs alleles and mutants of two genes whose products have known functions in wing development. (i) rhomboid, a component of the EGF-R signalling pathway, is expressed in vein cells and is required for specification of vein cell fate. rhove mutations (lacking rhomboid in wings) suppress the excess vein formation and associated with bs. Conversely, rho expression in prepupal and pupal bs wings is expanded in the regions of increased vein formation. (ii) The integrin genes, inflated and myospheroid, are expressed in intervein cells and are required for adhesion between the dorsal and ventral wing surfaces. Loss of integrin function results in intervein blisters. Integrin mutants interact with bs mutants to increase the frequency of intervein blisters but do not typically enhance vein defects. Both developmental and genetic analyses suggest that the bs product is required during metamorphosis for the initiation of intervein development and the concomitant inhibition of vein development.

Image manipulation. Confocal images to go?

Biologists are no longer restricted to using a single algorithm in their manipulation and display of data acquired using confocal microscopy.

Cis-acting regulatory sequences governing Wnt-1 expression in the developing mouse CNS

The protooncogene Wnt-1 encodes a short-range signal which is first expressed in, and appears to demarcate, the presumptive midbrain. Absence of Wnt-1 expression leads to the loss of this region of the brain. By the end of neural tube closure, expression of Wnt-1 extends down much of the dorsal midline of the central nervous system (CNS). Expression is exclusively limited to the CNS at this and later stages. We have investigated the regulation of Wnt-1 during mouse development. Analysis of the embryonic expression of Wnt-1-lacZ reporter constructs spanning nearly 30 kb of the Wnt-1 locus identified a 5.5 kb cis-acting 3′ enhancer element which confers correct temporal and spatial expression on the lacZ gene. Interestingly embryos express Wnt-1-lacZ transgenes in migrating neural crest cells which are derived from the dorsal CNS. Ectopic expression of the Wnt-1-lacZ transgenes may result from perdurance of beta-galactosidase activity in migrating neural crest cells originating from a Wnt-1-expressing region of the dorsal CNS. Alternatively, ectopic expression may arise from transient de novo activation of the transgenes in this cell population. These results are a first step towards addressing how regional cell signaling is established in the mammalian CNS. In addition, transgene expression provides a new tool for the analysis of neural crest development in normal and mutant mouse embryos.

Pattern formation and eyespot determination in butterfly wings

Butterfly wings display pattern elements of many types and colors. To identify the molecular processes underlying the generation of these patterns, several butterfly cognates of Drosophila appendage patterning genes have been cloned and their expression patterns have been analyzed. Butterfly wing patterns are organized by two spatial coordinate systems. One system specifies positional information with respect to the entire wing field and is conserved between fruit flies and butterflies. A second system, superimposed on the general system and involving several of the same genes, operates within each wing subdivision to elaborate discrete pattern elements. Eyespots, which form from discrete developmental organizers, are marked by Distal-less gene expression. These circular pattern elements appear to be generated by a process similar to, and perhaps evolved from, proximodistal pattern formation in insect appendages.

The role of apterous in the control of dorsoventral compartmentalization and PS integrin gene expression in the developing wing of Drosophila

During the development of Drosophila appendages from imaginal discs lineage restrictions appear that prevent dividing cells from crossing between regionally distinct compartments. These compartments correspond not only to regions of cell lineage restrictions but also to regions of specific gene expression. When compartments were first discovered, it was proposed that their formation relied on compartment-specific ‘selector’ gene activity; engrailed is thought to play such a role for the early-arising anterior-posterior restriction. Recent results suggest that the dorsally expressed transcription factor encoded by apterous may control dorsoventral identity in the wing. In this study we use mosaic analysis to show that apterous maintains the late-arising dorsoventral lineage restriction in a manner that strongly supports the selector gene hypothesis: loss of apterous function from dorsal cells after the formation of the boundary causes them to cross into the ventral compartment. Moreover, we show that apterous plays a role controlling patterns of gene expression in the developing wing disc. The PS1 and PS2 integrins are normally expressed in primarily dorsal-specific and ventral-specific patterns, respectively. We show that ectopic expression of apterous induces ectopic ventral expression of PS1 integrin and alpha PS1 mRNA, while loss of apterous can induce the ectopic dorsal expression of PS2 integrin. Thus, apterous plays a selector-like role both in terms of the control of lineage restrictions and the regulation of downstream gene expression.

Drosophila wingless: a paradigm for the function and mechanism of Wnt signaling

The link between oncogenesis and normal development is well illustrated by the study of the Wnt family of proteins. The first Wnt gene (int-1) was identified over a decade ago as a proto-oncogene, activated in response to proviral insertion of a mouse mammary tumor virus. Subsequently, the discovery that Drosophila wingless, a developmentally important gene, is homologous to int-1 supported the notion that int-1 may have a role in normal development. In the last few years it has been recognized that int-1 and Wingless belong to a large family of related glyco-proteins found in vertebrates and invertebrates. In recognition of this, members of this family have been renamed Wnts, an amalgam of int and Wingless. Investigation of Wnt genes in Xenopus and mouse indicates that Wnts have a role in cell proliferation, differentiation and body axis formation. Further analysis in Drosophila has revealed that Wingless function is required in several developmental processes in the embryo and imaginal discs. In addition, a genetic approach has identified some of the molecules required for the transmission and reception of the Wingless signal. We will review recent data which have contributed to our growing understanding of the function and mechanism of Drosophila Wingless signaling in cell fate determination, growth and specification of pattern.

To boldly glow ... applications of laser scanning confocal microscopy in developmental biology

The laser scanning confocal microscope (LSCM) is now established as an invaluable tool in developmental biology for improved light microscope imaging of fluorescently labelled eggs, embryos and developing tissues. The universal application of the LSCM in biomedical research has stimulated improvements to the microscopes themselves and the synthesis of novel probes for imaging biological structures and physiological processes. Moreover the ability of the LSCM to produce an optical series in perfect register has made computer 3-D reconstruction and analysis of light microscope images a practical option.

Interactions of decapentaplegic, wingless, and Distal-less in the Drosophila leg

The genes decapentaplegic, wingless, and Distalless appear to be instrumental in constructing the anatomy of the adult Drosophila leg. In order to investigate how these genes function and whether they act coordinately, we analyzed the leg phenotypes of the single mutants and their inter se double mutant compounds. In decapentaplegic the tarsi frequently exhibit dorsal deficiencies which suggest that the focus of gene action may reside dorsally rather than distally. In wingless the tarsal hinges are typically duplicated along with other dorsal structures, confirming that the hinges arise dorsally. The plane of symmetry in double-ventral duplications caused by decapentaplegic is virtually the same as the plane in double-dorsal duplications caused by wingless. It divides the fate map into two parts, each bisected by the dorsoventral axis. In the double mutant decapentaplegic wingless the most ventral and dorsal tarsal structures are missing, consistent with the notion that both gene products function as morphogens. In wingless Distal-less compounds the legs are severely truncated, indicating an important interaction between these genes. Distal-less and decapentaplegic manifest a relatively mild synergism when combined.

The function of the proneural genes achaete and scute in the spatio-temporal patterning of the adult labellar bristles of Drosophila melanogaster

The sensory precursors for labellar taste bristles develop from the labial disc in three distinct temporal waves occurring at 0 h, 8 h and 14 h of pupal development. In each temporal wave, transcripts for the achaete (ac) and scute (sc) genes are expressed in overlapping patterns in cells of the disc epithelium prior to the appearance of sensory mother cells (SMCs). No bristles form in mutant flies in which the ac and sc genes are absent. When the sc gene alone is deleted, a set of seven bristles fail to form. Pulses of ubiquitous sc ⁺ expression during pupal development, in a strain mutant for both ac and sc, can result in flies with all the labellar bristles at their correct positions. sc ⁺ pulses at times corresponding to the initiation of each of the waves of SMC specification in the disc was sufficient to restore bristle pattern. Bristles were not induced at ectopic positions and times as a result of the ubiquitous expression of sc ⁺. These results suggest that the proneural genes ac and sc do not themselves set the pattern of the labellar bristles. Instead, they are required for the elaboration of the pattern set by other gene products. We also show that the formation and positioning of the later waves of bristles can take place even in the absence of bristles normally specified earlier.

Organization of wing formation and induction of a wing-patterning gene at the dorsal/ventral compartment boundary

The appendages of arthropods and vertebrates possess a third, proximodistal patterning axis that is established after the primary anteroposterior and dorsoventral body axes by mechanisms that are largely unknown. The vestigial gene is required for formation of the entire Drosophila wing, and the dorsal/ventral boundary is shown to organize wing formation and vestigial gene expression. Interactions between dorsal and ventral cells in the growing imaginal disc induce vestigial gene expression through a discrete, extraordinarily conserved imaginal disc-specific enhancer. The link between dorsal/ventral compartmentalization and wing formation distinguishes the development of this sheet-like appendage from that of legs and antennae.

Compartment boundaries and the control of Drosophila limb pattern by hedgehog protein

Drosophila limbs are subdivided into anterior and posterior compartments which derive from adjacent cell populations founded early in development. Evidence is now provided that posterior cells organize growth and cell patterning in both compartments by secreting hedgehog protein and that hedgehog protein acts indirectly by inducing neighbouring anterior cells to secrete decapentaplegic or wingless protein.